RU2547882C2 - Method to measure medium temperature - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: method is proposed to measure medium temperature, according to which voltage drop is measured on a thermistor programmatically under management of a controller and on a reference resistor. The thermoelement and the reference resistor are connected in series to form a common electric circuit for passage of interrogate current. The platinum thermoelement of resistance is placed into an investigated medium. Interrogate current is generated as a periodical sequence of rectangular pulses with duty factor, in which average current via the thermoelement of resistance does not exceed the permissible value. The duty factor value is determined in accordance with the following formula: $$Q>\frac{Us}{(Rt+Rref)\times Iper},$$

where Q - required duty factor of pulse sequence, Us - value of DC voltage of the source of power supply that generates interrogate current of the thermoelement of resistance and reference resistor, Rt - value of thermoelement resistance at minimum measured temperature, Rref - rated value of resistance of the reference resistor, Iper - maximum permissible interrogate current of the thermoelement.

EFFECT: increased accuracy of temperature measurement.

1 dwg

Description

The invention relates to temperature measurements, and in particular to methods for measuring temperature with resistance thermal converters, and can be used to measure ambient temperature.

When measuring temperature with a resistance thermoconverter, as a rule, the problem arises of additional heating of the thermoconverter from the polling current flowing through it when measuring resistance. The presence of additional heating, due to the polling current, reduces the accuracy of measurements, since it leads to an increase in the error of temperature measurements, the value of which is difficult to take into account, since it has an unstable character. To minimize the influence of the magnitude of the polling current on the measurement accuracy for resistance thermocouples used to measure temperature, the permissible value of the current flowing around them is units of milliamps. However, in this case, accordingly, the voltage drop at the thermocouple, which you want to measure, is also small and amounts to about 0.5 V. There is a problem of ensuring the maximum level of the signal recorded from the thermocouple. To match the signal of the resistance thermoconverter and subsequent devices, such as an analog-to-digital converter, DC amplifiers are used.

A known method of measuring temperature, implemented in a device for measuring temperature, in which when changing the resistance of the thermocouple due to changes in the temperature of the controlled environment, they achieve the maximum signal level from the thermocouple at an acceptable dissipation power, automatically adjusting the magnitude of the polling current through a block of stable current sources, for which each stable the current source is configured to generate a given fixed polling current for a specific range of values th resistance of the thermal converter, the received signal is amplified (USSR, author certificate No. 1394062, G01K 7/00, 05/07/08).

The known method allows you to form the optimal operating mode of the thermal converter, receiving from it the maximum possible signal while maintaining the maximum allowable power dissipation. However, the signal from the thermal converter still remains small (fractions of a volt) and to ensure the operability of the device, additional signal amplification is required, which introduces an additional error in the temperature measurement results, since the output parameters of the amplifiers depend on many factors: stability of the power supply, environmental characteristics. Moreover, the error due to the use of signal amplifiers is difficult to take into account, since it is unstable. Minimizing the error introduced by the amplifier leads to a complication of the device that implements the method. In addition, the method of controlling the magnitude of the polling current is difficult to implement.

Closest to the proposed is a method for determining the temperature (RF, patent No. 2229692, G01K 7/18, 05/27/2004). Temperature measurement is performed programmatically under the control of a microcontroller. When measuring the temperature with a signal from the output, the microcontroller first connects the polling current source to a reference resistor, and then alternately to the resistance thermal converters. The measurement cycle begins with measuring the voltage drop across the reference resistor, which is amplified, the ADC is converted into a digital code and stored in the controller memory. Similarly, the voltage drop across the resistance thermocouples is alternately measured, while the switches alternately connect the thermocouples to the polling current source and to the amplifier. After the information from all resistance thermal converters arrives in the memory of the microcontroller, the voltage regulator is turned off by the microcontroller and the temperature of each test medium is sequentially calculated according to a previously defined algorithm.

The disadvantage of this method primarily consists in the fact that in it, as in the previous analogue, the signal from the thermal converter remains small (fractions of a volt), which requires additional signal amplification to ensure the operability of the method. The use of a signal amplifier in the measuring circuit introduces an additional error in the temperature measurement results, since the output parameters of the amplifier depend on many factors: the stability of the power source, and environmental characteristics. Moreover, the error due to the use of signal amplifiers is difficult to take into account, since it is unstable. Minimizing the error introduced by the amplifier leads to a complication of the device that implements the method.

In addition, in the known method, the measurement of the voltage drop across the reference resistor and resistance thermocouples is performed by alternately connecting a polling current source to them, i.e. when measuring the voltage drop, the interrogation current flows either only through the reference resistor, or only through the resistance thermal converter, i.e. these circuits are not electrically connected. Moreover, the equality of the magnitude of the polling current for the reference resistor and for thermal converters is provided by special hardware. This explains the need to use a stabilized constant voltage source. The need to use a stabilized source of constant voltage, forming the polling current, complicates the implementation of the claimed method.

The present invention solves the problem of a method for measuring the temperature of the medium, the implementation of which allows to achieve a technical result, consisting in the possibility of increasing the level of the signal taken from the resistance thermocouple without exceeding the permissible interrogation current, as well as to increase the accuracy of measurement and simplification.

The essence of the claimed invention lies in the fact that in the method of measuring the temperature of the medium, according to which a platinum resistance thermoconverter is placed in the test medium, a polling current is generated through a reference resistor and through a thermoconverter, programmatically under the control of the controller and in accordance with a predetermined algorithm, measure the voltage drop across a thermistor and a reference resistor, the measurement results are converted into a digital code, stored in the controller memory, the resistance value is determined a thermoconverter in the medium under study and the temperature of the medium is determined from the obtained value, it is new that the thermoconverter and the reference resistor are connected in series with the formation of a common electrical circuit for the polling current to flow, and the polling current flowing through them is formed in the form of a periodic sequence of rectangular pulses with a duty cycle, which the average current through the resistance thermoconverter does not exceed the permissible value, while the duty cycle is determined by the formula:

$$Q>\frac{U\mathrm{and}}{(Rt+R\mathrm{uh}t)\times Id\mathrm{about}P},$$

Where Q is the required duty cycle of the pulse sequence, Uand is the value of the constant voltage of the power source that forms the polling current of the resistance thermoconverter and the reference resistor, Rt is the resistance value of the thermocouple at the minimum measured temperature, Ret is the nominal resistance of the reference resistor, Idop is the maximum allowable current of the thermocouple interrogation.

The technical result is achieved as follows. Essential features of the claims of the claimed invention: “A method of measuring the temperature of a medium, according to which a platinum resistance thermoconverter is placed in the test medium, a polling current is generated through a reference resistor and through a thermoconverter programmatically controlled by the controller and in accordance with a predetermined algorithm, the voltage drop across the thermistor and reference resistor, the measurement results are converted into a digital code, stored in the controller memory, the resistance value t is determined rmopreobrazovatelya in the test medium and the resulting value is determined ambient temperature, ... "are an integral part of the claimed method and, together with the remaining essential features provide its implementation, and hence, provide the achievement of the claimed technical result.

In the claimed method, the thermal converter and the reference resistor are connected in series with the formation of a common electrical circuit for the flow of the polling current, in contrast to the prototype, where these circuits are not electrically connected. As a result, in the measuring circuit, the thermal converter and the reference resistance flow around the same current, the parameters of which for the reference resistor and the resistance thermal converter are not different, which eliminates the need for a stabilized constant voltage source.

In addition, the polling current is formed in the form of a periodic sequence of rectangular pulses, which makes it possible to supply a resistance temperature transducer and a voltage reference resistor for a short period of time equal to the pulse temperature, at which the value of the current flowing around them increases the controlled voltage drop across the thermal converter and the reference resistor, to a value that does not require additional gain. Moreover, due to the shape of a rectangular pulse, the value of the removed voltage drop will be constant. This eliminates the operation of amplifying an electrical signal taken from a thermal converter and a reference resistor.

In this case, since the polling current is formed in the form of a periodic sequence of rectangular pulses with a duty cycle at which the average current through the resistance thermoconverter does not exceed the permissible value, the signal level generated by the polling current on the thermoconverter increases without additional heating due to the flow of current through it a survey. The flow of polling current at the same time as the thermal converter and the reference resistor allows you to abandon the stabilized source of constant voltage to form the polling current.

In the claimed method, the magnitude of the duty cycle is determined by the formula:

$$Q>\frac{U\mathrm{and}}{(Rt+R\mathrm{uh}t)\times Id\mathrm{about}P},$$

Where Q is the required duty cycle of the pulse sequence, Uand is the constant voltage value of the power source that forms the polling current of the resistance thermal converter and the reference resistor, Rt is the resistance value of the thermal converter at the minimum measured temperature, Ret is the nominal resistance of the reference resistor, Idop is the maximum allowable polling current of the thermal converter, which makes it possible to generate a polling current from a single DC voltage source for thermocouples that are different from each other, taking into account admissible for each application mode, and also enables to form multichannel device for measurement of temperature with a wide range of measured temperatures.

In addition, the use of a mathematical formula for calculating simplifies the implementation of the method using software. This allows you to abandon the hardware implementation of the method in this part, thereby reducing the number of operations in the execution of the method.

As mentioned above, the parameters of electric signal amplifiers depend on many factors: the stability of the power source, environmental characteristics, which introduces an error in the measurement results, which is difficult to take into account, since it is unstable. Minimization of the hardware error introduced by the amplifier, complicates the device that implements the method, and complicates the method of measuring the temperature of the medium.

The exception from the method of the operation of amplification of the measured signal from the thermal converter and the reference resistor makes it possible to simplify the method and, therefore, to simplify the device implementing it by eliminating voltage amplifiers, as well as to improve the accuracy of temperature measurement, completely eliminating the analog processing of the measured signal from the resistance thermal converters, and eliminating the error introduced by the signal amplifier.

In addition, the ability to exclude from the claimed method the operation of amplification of the measured signal allows you to directly convert the measured signal into a digital code. Currently, the element base allows you to implement in one chip both the controller and the analog-to-digital converter, which further simplifies the implementation of the method.

Thus, from the foregoing, it follows that the claimed method of measuring the temperature of the medium during the implementation ensures the achievement of a technical result consisting in the possibility of increasing the level of the signal taken from the resistance thermocouple without exceeding the permissible interrogation current, as well as in improving the measurement accuracy and simplification.

Figure 1 shows a device for measuring the temperature of a medium that implements the claimed method of measuring the temperature of a medium, comprising a constant voltage source 1, an analog-to-digital converter 2 and a controller 3 connected by inputs / outputs, n resistance thermoconverters 4 ₁ -4 _n , a reference resistor 5 , n-channel DC switch 6, (n + 1)-channel switch 7 (hereinafter referred to as the channel switch), where n = 1, 2, 3, .... The DC voltage source 1 is connected by an output to the n-channel constant voltage switch 6, the first conclusions of the resistance thermocouples 4 ₁ -4 _{n are} connected to the corresponding outputs of the n-channel DC voltage switch 6 and to the corresponding n inputs of the channel switch 7, and the second conclusions are connected to reference resistor 5 and to the (n + 1) input of the channel switch 7. The output of the channel switch 7 is connected to the input of the analog-to-digital converter 2. The control inputs are respectively 8, 9 of the constant voltage 6 and channel switch 7 are connected respectively to the first 10 and second 11 control outputs of the controller 3.

There are no special requirements for the implementation of the controller and ADC, as well as for other circuit elements. The n-channel constant voltage switch can be made, for example, on bipolar transistors; (n + 1) -channel switch can be performed, for example, on a chip type 4051; resistance thermoconverter, for example, TPT-3-1; reference resistor - a precision resistor, for example, C2-29N.

The claimed method is as follows. The thermal converter and the reference resistor are connected in series with the formation of a common electrical circuit for the flow of polling current. A platinum resistance thermoconverter is placed in the test medium. Create a polling current flowing through a resistance thermoconverter and a reference resistor. Software under the control of the controller and in accordance with the specified algorithm, the voltage drop across the thermistor and on the reference resistor is measured, the measurement results are converted into a digital code, stored in the controller’s memory, the resistance value of the thermal converter in the medium under study is determined, and the medium temperature is determined from the obtained value. Moreover, the interrogation current flowing through the measuring circuit "thermocouple - reference resistor" is formed in the form of a periodic sequence of rectangular pulses with a duty cycle at which the average current through the resistance thermoconverter does not exceed the permissible value, while the duty cycle is determined by the formula:

$$Q>\frac{U\mathrm{and}}{(Rt+R\mathrm{uh}t)\times Id\mathrm{about}P},$$

Where Q is the required duty cycle of the pulse sequence, Uand is the value of the constant voltage of the power source that forms the polling current of the resistance thermoconverter and the reference resistor, Rt is the resistance value of the thermal converter at the minimum measured temperature, Ret is the nominal resistance of the reference resistor, Idop is the maximum allowable polling current of the thermal converter.

The method is implemented using a device for measuring temperature as follows. Management of the device implementing the inventive method is performed programmatically. The removal of information from temperature sensors - thermal converters of resistance 4 ₁ -4 _n and a reference resistor 5 - is performed alternately under the control of controller 3. After turning on the supply voltage, the control code from output 10 of controller 3 is received at the control input 8 of the n-channel DC voltage switch 6, which It connects to a source of DC voltage measuring circuit 1 specific "RTD - reference resistor", e.g., 4 ₁ 5. Simultaneously, the control input 9 of the channel switch 7 n the control code enters from the output 11 of controller 3 and connects the signal from the reference resistor 5 to the input of the analog-to-digital converter 2. A control code is present at the input 9 of the channel switch 7 during the measurement of the voltage drop across the reference resistor 5. The voltage drop across the reference resistor 5 is measured for each resistance thermoconverter. After measuring the voltage drop across the reference resistor 5, the code at the control input 9 of the channel switch 7 is changed so that the signal from the controlled resistance temperature converter 4 _{1 is} input to the analog-to-digital converter 2. At the end of the measurement, the codes from the control input 8 of the DC switch 6 and the control input 9 of the channel switch 7 are removed.

The control signal (code) from the outputs 10 and 11 of the controller 3 is a periodic sequence of rectangular pulses with a given duty cycle.

For each resistance thermoconverter, the duty cycle of pulses is pre-determined by the formula:

$$Q>\frac{U\mathrm{and}}{(Rt+R\mathrm{uh}t)\times Id\mathrm{about}P},$$

Where Q is the required duty cycle of the pulse sequence, Uand is the value of the constant voltage of the power source that forms the polling current of the resistance thermoconverter and the reference resistor, Rt is the resistance value of the thermal converter at the minimum measured temperature, Ret is the nominal resistance of the reference resistor, Idop is the maximum allowable polling current of the thermal converter.

The duty cycle of the control pulses is calculated so that when the power source is connected, the average current through the resistance temperature converter does not exceed the permissible value.

The following is an example of calculating the duty cycle for a TPT-3-1 type thermistor with a resistance value of Rt = 80 Ohms at the minimum measured temperature (-50 ° C), at 1 additional = 1 mA. For W = 5 V, R3T = 100 Ohms, we have:

$$Q>\frac{U\mathrm{and}}{(Rt+R\mathrm{uh}t)\times Id\mathrm{about}P}=5/(80+\mathrm{one\; hundred})\times \mathrm{one}\times {10}^{-3}\approx 28$$

The measurement of the voltage drop across the thermal converter and the reference resistor is performed over a period of time equal to the duration of the control pulse. For example, for a duty cycle of Q = 28 and a pulse repetition period of T = 2.8 s, the pulse duration τ will be 0.1 s (Q = T / τ).

In this case, the pulse current of the interrogation flows through the measuring circuit "resistance thermoconverter - reference resistor", which in a short period of time equal to the pulse duration exceeds the recommended value of the interrogation current. To simplify the calculation, let Icr = Idop = 1 × 10 ^-3 ;

Iimp = Iavg × Q = 1 × 10 ^-3 × 28 = 28 mA.

As a result, the voltage drop across the resistance thermoconverter and the reference resistor rises to a value that does not require additional amplification. In this case, the average value of the interrogation current through thermal converters does not exceed the permissible value.

After the controller receives information from the resistance thermoconverter and from the reference resistor, the temperature of each medium under study is calculated according to a predetermined algorithm. For example, the conversion of resistance into degrees is performed simultaneously with the linearization of the characteristics of the resistance thermal converter according to the table stored in the controller memory.

Claims (1)

The method of measuring the temperature of the medium, according to which a platinum resistance thermoconverter is placed in the test medium, a polling current is generated through the reference resistor and through the temperature converter, programmatically under the control of the controller and in accordance with the specified algorithm, the voltage drop across the thermistor and the reference resistor is measured, the measurement results are converted in a digital code, stored in the controller’s memory, the resistance value of the thermal converter in the medium under study and determined by the received the value is determined by the temperature of the medium, characterized in that the thermal converter and the reference resistor are connected in series with the formation of a common electrical circuit for the polling current to flow, and the polling current flowing through them is formed in the form of a periodic sequence of rectangular pulses with a duty cycle at which the average current through the thermal converter does not exceed permissible value, while the duty cycle is determined by the formula:
$$Q>\frac{U\mathrm{and}}{(Rt+R\mathrm{uh}t)\times Id\mathrm{about}P}$$

,
where Q is the required duty cycle of the pulse sequence, Uand is the constant voltage value of the power source that forms the polling current of the resistance thermoconverter and a reference resistor, Rt is the resistance value of the thermoconverter at the minimum measured temperature, RET - the nominal resistance of the reference resistor, Idop - the maximum allowable polling current of the thermal converter.